1. Field of the Invention
The present invention relates to an electronic device and a method for driving the same.
2. Description of the Background Art
In recent years, flat display panels have been used as display devices in a display unit of a notebook computer, a portable telephone, or a personal digital assistant (PDA). Many of such flat display panels are liquid crystal display devices. Since the power consumption is one of the factors that determine the product value of these portable devices, there is a demand for reducing the power consumption of a liquid crystal display device used in a display unit. Particularly, there is a demand for reducing the power consumption of the liquid crystal module itself in order to further reduce the power consumption of a reflection type liquid crystal display device that has no backlight.
Typically, the power consumption Pw of an electronic device including many capacitors such as a liquid crystal display device is expressed as Pw=Cxc2x7fxc2x7V2 based on a linear approximation, where C denotes the capacitance, f denotes the frequency, and V denotes the voltage. Therefore, the power consumption Pw can be reduced by reducing the capacitance C, the frequency f or the voltage V. Particularly, it is effective in reducing the power consumption to operate the device with a reduced voltage V because the power consumption Pw is proportional to the square of the voltage V as shown in the expression above, and because a voltage loss occurs when increasing the voltage supplied from the system (e.g., about 3.3 V in the case of a notebook computer).
In the prior art, the driving voltage of a liquid crystal display device has been reduced by reducing the threshold voltage of a liquid crystal layer, or setting the voltage for a gray level that requires the highest voltage to be lower than the normal voltage therefor, so as to narrow the dynamic range of the driving voltage.
However, such a low voltage driving operation has the following problems.
First, when using a liquid crystal material having a large (specific) dielectric anisotropy xcex94xcex5(=xcex5//xe2x88x92xcex5xe2x8axa5) in order to reduce the threshold voltage of the liquid crystal layer, the signal voltage dependence of a feed through voltage increases, whereby it is necessary to make a correction according to the magnitude of the signal voltage. When a liquid crystal material having a large (specific) dielectric anisotropy xcex5// in the long axis direction of liquid crystal molecules is used as a liquid crystal material having a large (specific) dielectric anisotropy xcex94xcex5, the liquid crystal capacitance is large, thereby increasing the size of TFTs (thin film transistors) for charging and increasing the load capacitance of the liquid crystal panel. Moreover, since the average dielectric constant of such a liquid crystal material is large, an impurity in the liquid crystal layer is easily ionized. Therefore, such a liquid crystal material undergoes a significant aging deterioration in terms of resistance, and thus cannot be used in a liquid crystal display device that is to be used under severe environments.
When the voltage for a gray level that requires the highest voltage is set to be lower than the normal voltage therefor, the contrast ratio is reduced, thereby reducing the display quality. Particularly, a liquid crystal display device that displays images in a normally white mode undergoes a significant reduction in the contrast ratio and thus a significant reduction in the display quality.
The present invention has been made in view of these problems, and has an object to provide an electronic device capable of operating with a low voltage and a method for driving the same.
An electronic device of the present invention includes on a substrate: a plurality of first capacitors arranged in a matrix pattern having rows and columns, each of the first capacitors including a first electrode and a second electrode opposing the first electrode via a first dielectric layer; a plurality of second capacitors provided so that there is one second capacitor at least for each row or each column, each of the second capacitors including a third electrode electrically connected to the first electrode and a fourth electrode opposing the third electrode via a second dielectric layer; a first line whose electrical connection to the first electrode and the third electrode is turned ON/OFF by a first switching element; a second line electrically connected to the second electrode at least temporarily; a third line whose electrical connection to the fourth electrode is turned ON/OFF by a second switching element; and a fourth line whose electrical connection to the fourth electrode is turned ON/OFF by a third switching element. Thus, the above object is realized.
In one embodiment of the invention: each of the plurality of first capacitors is a liquid crystal capacitor including the first electrode as a pixel electrode, the first dielectric layer as a liquid crystal layer, and the second electrode as a counter electrode opposing the pixel electrode via the liquid crystal layer; each of the plurality of second capacitors is a storage capacitor including the third electrode as a storage capacitor electrode, the second dielectric layer, and the fourth electrode as a storage capacitor counter electrode opposing the storage capacitor electrode via the second dielectric layer; and the liquid crystal layer modulates light passing therethrough according to a voltage applied between the pixel electrode and the counter electrode.
Preferably, the storage capacitor is provided so as to correspond to the liquid crystal capacitor. In one embodiment of the electronic device where the storage capacitor is provided so as to correspond to the liquid crystal capacitor, the first line is provided for each row or each column and functions also as the third line so as to supply a signal voltage to the pixel electrode, the storage capacitor electrode and the storage capacitor counter electrode, and the second line is provided for each row or each column and functions also as the fourth line so as to supply a counter voltage to the counter electrode and the storage capacitor counter electrode.
In one embodiment of the invention, the first line is provided for each row or each column and functions also as the third line so as to supply a signal voltage to the pixel electrode, the storage capacitor electrode and the storage capacitor counter electrode, the second line supplies a counter voltage to the counter electrode, the fourth line supplies the same voltage as the counter voltage to the storage capacitor counter electrode, and the storage capacitor is provided so as to correspond to the first line.
In one embodiment of the invention, the electronic device further includes a plurality of scanning lines provided so as to cross the first line for supplying a scanning signal to the first switching element, the second switching element and the third switching element.
Preferably, every adjacent two of the plurality of scanning lines form a scanning line pair, one of the two scanning lines, which form the scanning line pair, supplying a scanning signal to the first switching element and the third switching element with the other one supplying a scanning signal to the second switching element.
Preferably, the electronic device further includes a plurality of additional storage capacitors provided so as to correspond respectively to the plurality of liquid crystal capacitors, each of the additional storage capacitors including an additional storage capacitor electrode electrically connected to the pixel electrode and an additional storage capacitor counter electrode opposing the additional storage capacitor electrode via a third dielectric layer, wherein the third dielectric layer is formed from the same film as the second dielectric layer.
Preferably, the second switching element and the third switching element are transistors of different conductivity types.
A method of the present invention is a method for driving an electronic device, the electronic device including on a substrate: a plurality of first capacitors arranged in a matrix pattern having rows and columns, each of the first capacitors including a first electrode and a second electrode opposing the first electrode via a first dielectric layer; and a plurality of second capacitors provided so that there is one second capacitor at least for each row or each column, each of the second capacitors including a third electrode and a fourth electrode opposing the third electrode via a second dielectric layer, the method including the step of: switching a state where the first capacitor and the second capacitor are electrically connected in parallel to each other and another state where the first capacitor and the second capacitor are electrically connected in series with each other from one to another, thereby increasing a voltage being applied between the first electrode and the second electrode. Thus, the above object is realized.
Preferably, the voltage increasing step further includes the steps of: in the state where the first capacitor and the second capacitor are electrically connected in parallel to each other, applying a predetermined first potential to the first electrode and the third electrode while applying a predetermined second potential, which is different from the predetermined first potential, to the second electrode and the fourth electrode, so as to apply a predetermined voltage between the first electrode and the second electrode and between the third electrode and the fourth electrode, thus charging the first capacitor and the second capacitor; after charging the first capacitor and the second capacitor, achieving a state where the first electrode and the third electrode are electrically connected to each other and the first capacitor and the second capacitor are electrically connected in series with each other, while applying the predetermined second potential to the second electrode and applying the predetermined first potential to the fourth electrode, so as to increase the predetermined voltage applied between the first electrode and the second electrode; and after increasing the predetermined voltage, achieving a state where at least one of the second electrode and the fourth electrode is electrically cut off, whereby the increased voltage is held by the first capacitor.
In one embodiment of the invention: each of the plurality of first capacitors is a liquid crystal capacitor including the first electrode as a pixel electrode, the first dielectric layer as a liquid crystal layer, and the second electrode as a counter electrode opposing the pixel electrode via the liquid crystal layer; each of the plurality of second capacitors is a storage capacitor including the third electrode as a storage capacitor electrode, the second dielectric layer, and the fourth electrode as a storage capacitor counter electrode opposing the storage capacitor electrode via the second dielectric layer; and the liquid crystal layer modulates light passing therethrough according to a voltage applied between the pixel electrode and the counter electrode.
Preferably, the storage capacitor is provided so as to correspond to the liquid crystal capacitor.
In one embodiment of the invention, the storage capacitor is provided for each row or each column.